The mechanism by which the inner medulla of the kidney becomes hypertonic has been the subject of much speculation and mathematical modelling. Most of these proposals depend on observations made on in vitro perfused tubule segments. These present studies will attempt to define tubule properties in situ. Mathematical models require definition of the role of the vascular exchange bundles of the medulla. It is hoped in the present studies to determine the elemental gradients in and along these structures. The electronprobe microanalyzer, by virtue of its high sensitivity (10 to the minus 16th power gm) and spatial resolution (approximately 1 micron) provides a powerful tool for elemental analysis. By using it in conjunction with microchemical, histochemical, and fluorometric techniques it is hoped that hitherto inaccessible regions of the kidney may be subjected to direct analysis. This project seeks to study the mechanism of urine concentration using the techniques of electronprobe microanalysis. The composition of tubular fluids will be measured in ice removed from frozen sections of rat kidneys. Elemental composition and regional gradients, as well as the distribution of Na-K-ATPase will be defined and mapped both in the inner and outer medulla.